1. Field of the Invention
The present invention concerns the control of the attitude or pointing direction of artificial earth satellites.
2. Description of Related Art
Conventionally the attitude of a satellite is determined using attitude sensors which may take the form of optical or inertial sensors. Optical sensors utilise radiation emanating from a selected reference such as the sun, earth, moon or stars to output signals relating to the attitude of the satellite. Inertial sensors used in satellites are normally precision gyroscopes.
Inertial sensors have an advantage in respect of optical sensors in that they can often provide a superior signal to noise ratio over the majority of the frequency range of interest for satellite attitude sensing. This range is normally D.C. to 10 Hz. However they suffer from drift at the D.C. end of this range whereas optical sensors do not. On the other hand optical sensors suffer from the disadvantage that they can output unwanted noise as a contamination of the output signal either due to the weakness of the radiation being sensed or to the level of ambient electromagnetic disturbance in the complex and compact environment of the satellite.
A further disadvantage of inertial sensors is that at the high end of the frequency range of interest the output signals of even inertial sensors can be contaminated by an unacceptable level of noise. In the instance of gyroscopes this noise arises due to vibrations and variabilities introduced by the rotation of the gyroscope wheel and its ball races which can produce a variety of periodic disturbances in the 1 to 100 Hz region.
One attempt to overcome these disadvantages has been to use both inertial and optical sensors in a single attitude control system. In this known control system the outputs of the two types of sensor are combined in a Kalman filter. In order to improve performance it has also been proposed to feed into the Kalman filter the control signals used to control the actuators for altering the attitude of the satellite. Such actuators are conventionally thrusters or reaction wheels. Where such feedback of the actuator control signals has been used it has been found feasible to use a system which dispenses with the expensive inertial sensor. However even the use of the actuator control signals as feedback presents problems in that the degree of improvement available in the control of attitude by using this technique to dependent on the relationship between the control signals fed to the actuators which actually control satellite attitude and the actual response of the actuator in terms of its thrust or torque profile.